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Hole traps in p-type Cu 2 O were studied by means of deep level transient spectroscopy in the heterostructure of p-Cu 2 O/i-ZnO/ n-ZnO. In addition to the trap level at about 0.45 eV from the valance band edge, which is already reported as being due to Cu vacancy, we found a new trap level at about 0.25 eV. The new trap is tentatively assigned as Cu-di-vacancy from the trap concentration dependence on oxygen flow rate and substrate temperature. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2175492͔ Cuprous oxide ͑Cu 2 O͒, a direct band gap semiconductor with a band gap energy of 2.0 eV, has been regarded as one of the most promising materials for application to photovoltaic cells, 1,2 especially for the top cell in a tandem structure. The attractiveness of Cu 2 O as a photovoltaic material lies in the fact that the constituent materials are nontoxic, low cost and abundantly available. Cu 2 O / ZnO heterojunction has been fabricated by radio frequency ͑rf͒ magnetron sputtering and showed the photovoltaic effects, but did not demonstrate good performance. 3 Knowledge of defect energies as well as their densities is an important input for further improvement of the performance of Cu 2 O thin film polycrystalline solar cells. There are several reports on the deep trap in Cu 2 O by measuring deep level transient spectroscopy ͑DLTS͒. 4-6 A hole trap with the activation energy of about 0.45 eV from the valence band edge has been observed and assigned as Cu vacancy using Schottky diodes. However, the temperature range in the DLTS measurements was too narrow. It is necessary to measure the DLTS in the expanded temperature range in order to understand the origin of the defect in Cu 2 O. In this work, the DLTS spectra of Cu 2 O with the junction of n-ZnO/ i-ZnO/ p-Cu 2 O structure in the temperature range between 100 and 350 K is reported. We observed a new trap with the activation energy of 0.25 eV from the valence band edge in addition to the 0.45 eV trap, and the origin of the 0.25 eV trap is discussed based on the sample preparation conditions.Polycrystalline p-Cu 2 O/n-ZnO heterostructure was grown by means of rf magnetron sputtering on Corning 7059 glass substrate using a Cu target of 99.99% purity, ZnO target and Ar as sputtering gas. Oxygen was introduced during the growth of Cu 2 O through a nozzle whose end was placed near the substrate. We prepared two sets of samples for Cu 2